MISTER 880 is an elderly gentleman, Skipper Miller (Edmund Gwenn), who in his spare time has taken up counterfeiting to supplement his income. His brazen amateurishness, however, raises the ire of the treasury department. One treasury agent in particular, the hard-nosed Steve Buchanan (Burt Lancaster), decides to make it his mission to track down and prosecute the "mastermind" behind the counterfeiting. One of his leads takes him to Ann Winslow (Dorothy McGuire), who has received one of the phony dollar bills. The hardened Buchanan is softened by Ann's winsomeness and he begins to fall in love with her. Putting together the information he has been able to come up with, he discovers that Skipper lives in Ann's building. MISTER 880 is based on a true story, and derives its cryptic title from the number of the Secret Service file upon which the film is based.